Nonthermal hard X-ray emission from the Galactic Ridge

Abstract

We investigate the origin of the nonthermal X-ray emission from the Galactic ridge in the range 10-200 keV. We consider bremsstrahlung of subrelativistic cosmic ray protons and electrons as production processes. From the solution of the kinetic equations describing the processes of particle in-situ acceleration and spatial propagation we derive parameters of the spectra for protons and electrons. For in-situ acceleration the flux of accelerated particles consists mainly of protons since the ratio of the accelerated protons to electrons is large and the flux of nuclei with charges Z>1 is strongly suppressed. Our analysis has shown that the proton bremsstrahlung model is free from the previously discussed problems of the high gamma-ray flux at MeV or at hundreds of MeV energy regions. However, the model with proton bremsstrahlung requires specific restrictions on the proton spectrum.The real problem of the proton bremsstrahlung model is the large pressure of the accelerated protons in the disk. For the electron origin of the ridge flux we do not have the problems of the gamma-ray line and continuous fluxes. On the other hand the electron bremsstrahlung model requires more effective acceleration of background particles.